1. Field
The invention relates generally to a method of measuring blood pressure, and more particularly to a method of reducing the effects of noise in the light signal of a PPG blood pressure measuring system.
2. Description of Related Art
Pulse contour analysis (PCA) is the process of calculating parameters from a blood pressure pulse, especially from the contour of the pulse wave. PCA begins with measuring blood pressure (BP).
Blood pressure may be measured in a number of ways. As one example, a standard non-invasive sphygmomanometer (NBP) may be placed on the upper arm or wrist. The NBP applies pressure to the arteries, causing them to constrict and limit blood flow. As the pressure is released, blood flow is restored in the artery, and the systolic and diastolic blood pressures may be measured. NBP measures BP intermittently and not continuously, so it cannot be used for PCA.
Another device for measuring blood pressure is a finger cuff having an infrared light source and a light detector for measuring a photo-plethysmographic (PPG) signal that is known also from pulsoximetry. This PPG-signal is fed into a control system, which produces a counter pressure in the finger cuff. The counter pressure equals intra-arterial pressure when the PPG-signal is kept constant. Thus, the counter pressure, which is indirectly equivalent to intra-arterial pressure, is measured. This method is known as “Vascular Unloading Technique” and the continuous pressure signal can be used for PCA.
Invasive devices may also be used to measure blood pressure, such as an intra-arterial catheter, for example. Intra-arterial transducers have relatively high frequency transmission (up to 200 Hz) and can therefore be used for PCA.
Some example parameters that may be calculated from the contour of the pulse wave include stroke volume (SV), cardiac output (CO), stroke volume variation (SVV), pulse pressure variation (PPV), and total peripheral resistance (TPR). In addition, PCA can be used for other measurements which give insight to the human vascular properties, for example arterial stiffness. Thus, it is desirable that the measured blood pressure signals be as accurate as possible.
Invasive devices have the disadvantage of being overly disturbing and painful to the patient, whereas signals from non-invasive devices have problems with the fidelity or accuracy of the signal.